The present invention relates to a method of forming a semiconductor structure in a substrate comprising insulating layers with a different thickness and a semiconductor structure formed therewith.
Semiconductor structures can comprise insulating layers, with a different thickness, in order to fulfill certain tasks. For example, technologies for integrated circuits (IC) offering high voltage vertical trench double diffused metal oxide semiconductor (TDMOS) as power transistors, need a thick insulating layer, for example, an oxide layer on the bottom and the side walls of the trenches. This thick oxide layer, here named also as field plate oxide (FPOX) can be thermally grown at a high temperature. After growth, the FPOX can be structured by applying additional processes, so that it remains on the bottom and the side walls of the trenches of the central part of the cell field trenches and all over inside the peripheral trenches. At the same time, another thick oxide on the planar part of the semiconductor structure should be present, this thick oxide is here also named field oxide (FOX). The field oxide can be grown at a lower temperature than the field plate oxide. This is compatible with a previous implantation of the semiconductor structure, which is needed to ensure the correct functionality of the structure. The implanted region can fulfill a certain task, for example, it can act a channel stop-zone for a parasitic depletion MOSFET. A n-type implantation for example can be used for a depletion n-channel MOSFET formed in the substrate. The field plate oxide and the field oxide have their own optimal growth conditions and final thickness. Normally, the desired thickness of the field oxide is greater than the thickness of the field plate oxide.
For the fabrication of such a semiconductor structure, e.g. a TDMOS, comprising insulating layers with different thickness, it is desirable to reduce the number of mask steps for cost and yield reasons and to ensure, at the same time, the proper functionality of other semiconductor structure parameters. For the TDMOS transistors a soft topography of a boron-phosphorus-silicate-glass (BPSG) layer covering the field oxide should be for example also ensured.